Age changes were demonstrated in the human stellate ganglion, a sympathetic ganglion. These included a loss of cytoplasmic catecholamine fluorescence, dendritic hypertrophy and neuroaxonal dystrophy. In adult paravertebral human sympathetic ganglia, immunoreactivity to the following peptides [Met5]enkephalin, [Met5]enkephalin-Arg6-Phe7 and bombesin-gastrin releasing peptide, as well as Substance P, was localized in varicose nerve fibers, but not in cell bodies. Increased secretion by autonomic paraganglia is thought to contribute to elevated plasma catecholamines levels in senescent Fischer-344 rats. In such paraganglia, there was a progressive increase, by up to 10 fold, in the number of cells between 3 and 24 months of age, and in tyrosine hydroxylase and choline acetyltransferase activities, enzymes necessary for synthesis of catecholamines and acetylcholine, respectively. Adrenalectomy did not affect choline acetyltransferase but reduced tyrosine hydroxylase activity by 60% in senescent rats. Age changes in cell number and catecholamine fluorescence of paraganglia suggested an active endocrine, secretory function in the older Fischer-344 rats. The catecholamine synthesizing enzymes, tyrosine hydroxylase, dopamine-Beta-hydroxylase and phenylethanolamine N-methyltransferase were demonstrated by immunohistochemical techniques.